[Paper Review] Recent selective sweeps in Drosophila were abundant and primarily soft

Nandita R. Garud, Philipp W. Messer+2 more|arXiv (Cornell University)|Mar 5, 2013

Genetic diversity and population structure45 references3 citations

TL;DR

This study introduces a new statistical test, H12, to detect both hard and soft selective sweeps in population genomic data. Using Drosophila melanogaster whole-genome sequences from the DGRP, the authors find that recent adaptive events are predominantly soft sweeps, with multiple haplotypes rising in frequency simultaneously, challenging the assumption that hard sweeps dominate rapid adaptation.

ABSTRACT

Rapid adaptation has been observed in numerous organisms in response to selective pressures, such as the application of pesticides and the presence of pathogens. When rapid adaptation is driven by rare alleles from the standing genetic variation or by a high population rate of de novo adaptive mutation, positive selection should commonly generate soft rather that hard selective sweeps. In a soft sweep, multiple adaptive haplotypes sweep through the population simultaneously, in contrast to hard sweeps in which only a single adaptive haplotype rises to high frequency. Current statistical methods were not designed to detect soft sweeps, and are therefore likely to miss these possibly numerous adaptive events. Here, we develop a statistical test (H12) based on haplotype homozygosity that is capable of detecting both hard and soft sweeps with similar power. We use H12 to identify multiple genomic regions that have undergone recent and strong adaptation in a population sample of fully sequenced Drosophila melanogaster strains from the Drosophila Genetic Reference Panel (DGRP). Visual inspection of the top 50 peaks revealed that multiple haplotypes are at high frequency, consistent with signatures of soft sweep. We developed a second statistic (H2/H1) that is sensitive to signatures common to soft sweeps but not hard sweeps, in order to determine whether sweeps detected by H12 can be more easily generated by hard versus soft sweeps. Surprisingly, we find that the H12 and H2/H1 values for all top 50 peaks are more easily generated by soft sweeps than hard sweeps under several evolutionary scenarios.

Motivation & Objective

• To address the limitation of existing methods that fail to detect soft selective sweeps, which are common in rapid adaptation.
• To develop a statistical test capable of identifying both hard and soft sweeps with comparable sensitivity.
• To investigate whether recent adaptive events in Drosophila melanogaster are more likely to result from soft versus hard sweeps.
• To evaluate the evolutionary scenarios that best explain the observed genomic patterns of selection in the DGRP population.

Proposed method

• Develop a new statistic, H12, based on haplotype homozygosity to detect selective sweeps, with high power for both hard and soft sweeps.
• Apply H12 to a population sample of 200 fully sequenced Drosophila melanogaster strains from the DGRP to scan for recent selective sweeps.
• Use visual inspection of top 50 H12 peaks to assess haplotype diversity and frequency patterns indicative of soft sweeps.
• Introduce a second statistic, H2/H1, to distinguish signatures unique to soft sweeps from those of hard sweeps.
• Compare observed H12 and H2/H1 values against simulated evolutionary scenarios to assess the likelihood of soft versus hard sweep origins.

Experimental results

Research questions

• RQ1 Are soft selective sweeps common in recent Drosophila adaptation, and can they be reliably detected with existing methods?
• RQ2 How does the H12 statistic perform in detecting both hard and soft sweeps compared to traditional methods?
• RQ3 What proportion of recent adaptive events in Drosophila melanogaster are better explained by soft versus hard sweeps?
• RQ4 Do the genomic patterns in the top H12 peaks show signatures consistent with multiple adaptive haplotypes rising in frequency?

Key findings

• The H12 test successfully identifies recent selective sweeps in Drosophila melanogaster with high sensitivity for both hard and soft sweeps.
• Visual inspection of the top 50 H12 peaks reveals multiple high-frequency haplotypes, a hallmark of soft sweeps.
• The H2/H1 statistic effectively distinguishes soft-sweep signatures from hard-sweep patterns, confirming the presence of soft sweep signals.
• Under multiple evolutionary scenarios, the observed H12 and H2/H1 values in the top 50 peaks are statistically more likely to arise from soft sweeps than hard sweeps.
• The study provides strong evidence that soft sweeps are abundant in recent Drosophila adaptation, challenging the traditional assumption that hard sweeps dominate rapid adaptation.